The present invention relates to a method for forming images using photosensitive materials of the type described in U.S. Pat. Nos. 4,399,209 and 4,440,846.
U.S. Pat. No. 4,399,209 to The Mead Corporation describes a transfer imaging system wherein an imaging sheet comprising a support and a layer of microcapsules containing a chromogenic material and a photosensitive composition is image-wise exposed to actinic radiation. The exposed sheet is assembled with a developer sheet and the two sheets are passed together through a calendar nip whereupon the microcapsules rupture and the contents of the microcapsules are image-wise transferred to the developer sheet where the chromogenic material reacts to form a colored image. Typically, the photosensitive composition is a radiation curable composition containing an ethylenically unsaturated compound and a photoinitiator, the chromogenic material is a substantially colorless electron donating compound, and the developer is an electron acceptor. Upon image-wise exposing the imaging sheet to actinic radiation, the photosensitive composition hardens in the exposed area but remains liquid in the unexposed areas. Thus, the microcapsules in the unexposed areas remain capable of rupturing and releasing the chromogenic material upon subjecting the imaging sheet to a uniform rupturing force whereas the microcapsules in the exposed areas do not.
Related U.S. application Ser. No. 339,917 filed Jan. 18, 1982, corresponding to U.K. Pat. No. 2,113,860, describes an imaging sheet and method useful in the preparation of full color images. The application discloses both ultraviolet sensitive and visible light sensitive imaging sheets. The most typical imaging sheet prepared in accordance with the teachings of this application includes a layer containing three sets of microcapsule homogeneously distributed on the surface of the support. Each set of microcapsules contains a different radiation curable composition which is principally sensitive to a different band of actinic radiation. In accordance with one embodiment, the three bands of radiation are in the 300-400 nm range. See U.S. Pat. No. 4,576,891. In accordance with another embodiment, the three distinct bands of radiation are red, green and blue light. Exposure of the three sets of microcapsules controls the release of cyan, magenta and yellow image-forming agents. Upon subjecting the exposed imaging sheet to a uniform rupturing force in contact with the developer sheet, the cyan, magenta and yellow image-forming agents are image-wise transferred to the developer sheet where a full color image is formed.
U.S. application Ser. No. 944,305, filed Dec. 18, 1986 corresponding to Published European Application No. 0233587 discloses a panchromatic imaging material.
Various methods have been proposed for image-wise exposing the aforementioned imaging sheets, and more particularly, imaging sheets useful in the production of full color images. The simplest approach involves the preparation of color separations which are assembled with the imaging sheet for exposure to three distinct bands of radiation. Another method which has been proposed relies the use of liquid crystal or light valve technology. The liquid crystal selectively transmits light to the surface of the imaging sheet for exposure. Thus, by controlling the liquid crystal, the liquid crystal functions analogous to a color separation.
In each of the aforementioned methods, the imaging sheet is exposed by selectively transmitting radiation to the surface of the imaging sheet. The amount of chromogenic material released from a predetermined microcapsule is a function of the amount of exposure which the microcapsule receive. Microcapsules receiving a high amount of exposure (fully exposed microcapsules) do not release the chromogenic material. Microcapsules receiving a low amount of exposure or no exposure (unexposed microcapsules) release the chromogenic material fully. Microcapsules receiving an intermediate degree of exposure may release the chromogenic material in an intermediate amount.
Japanese Patent Publication No. 62-60694 discloses a methods of imaging wherein an imaging sheet analogous to that described in U.S. Pat. No. 4,399,209 is image-wise heated and simultaneously uniformly exposed to light or ultraviolet radiation to form images. The internal phase of the microcapsules includes a radiation sensitive composition having a temperature (e.g., a glass transition temperature) above which the composition must be heated before it reacts efficiently in the presence of light. In the absence of heat, the composition reacts very slowly. Accordingly, by image-wise heating this imaging sheet while simultaneously exposing the imaging sheet to light, images can be formed. In the heat-exposed areas, the radiation sensitive composition hardens such that the microcapsule does not rupture and release the image-forming agent upon subjecting the microcapsule to a uniform rupturing force. In the heatunexposed areas, light does not substantially alter the properties of the radiation sensitive composition and, consequently, the microcapsules remain capable of releasing the image-forming agent. This method is advantageous because images can be formed in the imaging sheet through the use of a thermal printhead instead of optical means such as color separations, liquid crystals, etc.